Socially and professionally, most people rely upon video displays in one form or another for at least a portion of their work and/or recreation. With a growing demand for large screens and high definition television (HDTV), cathode ray tubes (CRT's) have largely given way to displays composed of liquid crystal devices (LCDs), light-emitting diodes (LEDs), plasma and front and rear projection systems.
A CRT operates by a scanning electron beam exciting phosphorous-based materials on the back side of a transparent screen, wherein the intensity of each pixel is commonly tied to the intensity of the electron beam. With an LED and plasma display, each pixel is an individual light-emitting device capable of generating its own light. With an LCD display, each pixel is a transient light-emitting device, individually adjusted to permit light to shine through, or reflect through the pixel by altering the polarization of the transmitted or reflected light.
As LCD, plasma and LED screens do not utilize a large tube, as in a CRT, LCD, plasma and LED screens may be quite thin and in many cases are lighter than comparable CRT displays. As such large and small flat screen displays can be provided to improve the portability of laptop computers, video displays in vehicles and airplanes, and information displays that are mounted or set in eye catching locations.
Typically incorporated into the screens of such flat screen devices as LCD, plasma and LED displays are a plurality of thin film devices, such as transistors. Specifically, one or more transistors are commonly used to control the behavior of each pixel within the display. The individual nature of each pixel of a LED, plasma or LCD display introduces the possibility that each pixel may not provide the same quantity of light. One pixel may be brighter or darker than another, a difference that may be quite apparent to the viewer.
As a flat screen display may incorporate hundreds of thousands of transistors, great care is generally applied in the fabrication of LED, plasma and LCD displays in an attempt to ensure that the pixels (and more specifically the backplane transistors controlling the pixels) are as uniform and consistently alike as is possible. Frequently, especially with large displays, quality control measures discard a high percentage of displays before they are fully assembled. As such, displays are generally more expensive than they otherwise might be, as the manufacturers must recoup the costs for resources, time and precise tooling for the acceptable displays as well as the unacceptable displays.
Traditionally, thin film devices have been formed through processes such as photolithography. In a photolithographic process, a substrate is provided and at least one material layer is uniformly deposited upon the substrate. A photo-resist layer, also commonly known simply as a photoresist, or even resist, is deposited upon the material layer, typically by a spin coating machine. A mask is then placed over the photo resist and light, typically ultra-violet (UV) light, is applied. During the process of exposure, the photoresist undergoes a chemical reaction. Generally the photoresist will react in one of two ways.
With a positive photoresist, UV light changes the chemical structure of the photoresist so that it is soluble in a developer. What “shows” therefore goes, and the mask provides an exact copy of the patterns which are to remain—such as, for example the trace lines of a circuit.
A negative photoresist behaves in the opposite manner—the UV exposure causes it to polymerize and not dissolve in the presence of a developer. As such the mask is a photographic negative of the pattern to be left. Following the developing with either a negative or positive photoresist, blocks of photoresist remain. These blocks may be used to protect portions of the original material layer, serve as isolators or other components.
Very commonly, these blocks serve as templates during an etching process, wherein the exposed portions of the material layer are removed, such as for example, to establish a plurality of conductive rows.
The process may be repeated several times to provide the desired thin film devices. As such, new material layers are set down on layers that have undergone processing. Such processing may inadvertently leave surface defects in the prior layers as well as unintended contaminant particles.
The crystalline texture of the materials composing each material layer, and specifically the crystalline texture of each material at an interface between materials is often of significant importance to the operation of the thin film device. Surface defects and surface contaminants may negatively affect the interfaces between layers and possibly degrade the performance of the thin film device.
In addition, photolithography is a precise process applied to small substrates. In part this is due to the high cost of the photo masks. For the fabrication of larger devices, typically rather than employing a larger and even more costly photo mask, a smaller mask is repeatedly used—a process that requires precise alignment.
As a photolithographic process typically involves multiple applications of materials, repeated masking and etching, issues of alignment between the thin film layers is of high importance. A photolithographic process is not well suited for formation of thin film devices on flexible substrates, where expansion, contraction or compression of the substrate may result in significant misalignment between material layers, thereby leading to inoperable thin film devices. In addition a flexible substrate is not flat—it is difficult to hold flat during the imprinting process and thickness and surface roughness typically can not be controlled as well as they can for glass or other non-flexible substrates.
The issue of flatness in photolithography can be problem because the minimum feature size that can be produced by a given imaging system is proportional to the wavelength of the illumination divided by the numerical aperture of the imaging system. However the depth of field of the imaging system is proportional to the wavelength of the illumination divided by the square of the numerical aperture. Therefore as resolution is increased the flatness of the substrate quickly becomes the critical issue.
With respect to the flat screen displays introduced above, use of flexible substrates for the internal backplane controlling the pixels is often desired. Such a flexible substrate can provide a display with flexible characteristics. A flexible substrate may also be easier to handle during fabrication and provide a more mechanically robust display for the user.
Hence, there is a need for a process to provide at least one thin film device that overcomes one or more of the drawbacks identified above.